WHY only 2008 tasks for many consecutive months and WHY do 20% of these automatically error "100.000 Computation error (0.244 CPUs + 1 AMD/ATI GPU)" immediately after downloading? If I suspend GPU, same computation error still occurs but then I must manually abort each to get them off the window. Is this fixable or normal or am I caught in a loop?

The Project can't just exclude your ATi4 GPU without excluding All the Mac GPUs, so I'm told.
The Best action would be to manually Exclude your ATi4 from SETI, that way you could possibly use the GPU somewhere else. You do that with a cc_config.xml file in your BOINC Data folder.
You need to use a BOINC compatible .xml file, the easiest way to get one of those if you don't have one is to Duplicate an existing BOINC .xml file and then change the name and replace the contents.
The cc_config.xml file needs to be in /Library/Application Support/BOINC Data. Go there and Duplicate global_prefs.xml using TextEdit, change the name of the Duplicate to cc_config.xml and replace the contents with the following;

Thank you TBar! ...for taking your time to make this so simple for me. I'm not software or even script literate. Your easy instructions completed flawlessly and I will know if this fixes my ATI4 GPU computation error problem after the next few grouped task downloads over the next 24 hours. I will update here immediately after I know for sure.

You're right Mike, and even though I previously ruled out installing a HD 5000 for cost vs performance reasons, I am now faced with replacing all the fans in this aging Mac that has been crunching 24/7 since 2009. So, while it's all opened up on the bench, I might just do the GPU upgrade anyway. I might even switch to a SSD as I've already had one HDD failures since 2009.

TBar [UPDATE]...
About two hours after I made these changes and restarted BOINC Manager, nine more GPU computation errors occurred. Is this something that may take more time to work or did these changes not fix the problem? Also, I still have "Use GPU always" selected if that makes any difference. Oops [UPDATE 2]... disregard first update, I discovered and fixed a typo that I made in the first line of your script. Restarted and continuing with the test.

It's always best to use copy and paste, although I have seen errors using that method. To see if the file is being used look in the Event Log after startup and see if the GPU is being Excluded.
I wouldn't bother with the GPU update in that machine, the updated GPU would still be very slow. If you could, a better result would be to Swap the machine for a Classic Mac Pro where you could run a recent nVidia card with the CUDA Special App. Just One GTX 950 would give these results, https://setiathome.berkeley.edu/show_host_detail.php?hostid=4726043
My machine was running 3 GTX 950s, it's just slightly better with the 1050s, https://setiathome.berkeley.edu/show_host_detail.php?hostid=6796479
It really doesn't matter if it can't run the Current OS, it runs the SETI tasks fine. I'll be posting the App for the Pascal cards soon, seems the CUDA 75 App still doesn't work quite right with the Pascal drivers while the CUDA 80 App seems to work fine.

The Project can't just exclude your ATi4 GPU without excluding All the Mac GPUs, so I'm told.
The Best action would be to manually Exclude your ATi4 from SETI, that way you could possibly use the GPU somewhere else. You do that with a cc_config.xml file in your BOINC Data folder.
You need to use a BOINC compatible .xml file, the easiest way to get one of those if you don't have one is to Duplicate an existing BOINC .xml file and then change the name and replace the contents.
The cc_config.xml file needs to be in /Library/Application Support/BOINC Data. Go there and Duplicate global_prefs.xml using TextEdit, change the name of the Duplicate to cc_config.xml and replace the contents with the following;